Can microbiology tell us exactly what killed the Aztecs?

The arrival of the Spanish conquistadors to Mexico in the 1620s marked the beginning of the end for the indigenous people. With an estimated population of between 15 and 30 million at this point, this dropped dramatically to only two million by 1700: the result of battles, famine, drought, and perhaps most significantly, infectious diseases. The following Q&A investigates how microbiology contributed to the ruin of the once-flourishing Mesoamerican culture.

What is the ‘cocoliztli’ and how was it introduced to the Aztec population?

Previously, historians have established that the epidemics of 1545-1548 and 1576-1580 – also known as the ‘cocoliztli’, or ‘plague’ in the native Aztec language náhuatl – were imported by the Spaniards during their arrival. The indigenous population, who had no previous immunity against the newly introduced viruses, quickly succumbed to the diseases. Alongside these epidemics, the revolution of the social order, evangelization, destruction of ancient culture, and architectonic transformation all contributed to the start of the Colonial Era in Mexico.

When was the ‘cocoliztli’ first clinically documented?

In 1576, Spanish physician and naturalist Francisco Hernández and physician Alfonso Lopez de Hinojosos described the ‘cocoliztli’. In their account, they detailed a clinical condition that was highly contagious, although different from other diseases of the time such as smallpox, measles, and typhus. Symptoms of ‘cocoliztli’ included severe weakness, strong headaches, dry mouth, dizziness, and stomach pain and haemorrhages. They included that death could occur just four or five days after the appearance of the initial symptoms. Thus, this plague was referred to as haemorrhagic fever, or blood congestion.

Indigenous victims, Florentine Codex (compiled 1540–1585). As reproduced in: Fields, Sherry (2008). Public domain via Wikimedia Commons.

How has microbiology evolved to give us a better understanding of ancient epidemics?

In recent years, state-of-the-art technologies have allowed scientists to learn much more about today’s microbial world. By extension, these same technologies could allow insight into the causal agent of epidemics which struck centuries ago. Just one example of this is a 1998 study where teeth pulp was extracted from what were believed to be the remains of victims of the Bubonic Plague. As a highly vascularized tissue, it is conceivable that blood in the pulp – contaminated with bacteria, viruses, or both – could provide insight into the various pathogens that circulated in the bodies of long-ago diseased individuals. The detection of Yersinia pestis DNA sequences from the teeth pulp in this study was the first step in discovering the microbial history of the Black Death which decimated the European population in 1347-1351.

How do these developments help us pinpoint the causal agent of the ‘cocoliztli’?

Similar to the 1998 study, investigators extracted dental pulp from individuals buried in a cemetery associated with the ‘cocoliztli’, which led to the reconstruction of two causal agent genomes. These genomes were identified as the Paratyphi C strain of the pathogen Salmonella enterica, a causal agent of enteric fever, or typhoid fever, suggesting that this bacterium could have been involved in the Aztec plague. Known to be facilitated by the faecal-oral route, this species would have thrived in the poor sanitary conditions in Colonial New Spain, with the indigenous people immunologically defenceless against the infectious diseases of the Old Continent. Another study performed on ancestral DNA from skeletons found in Trondheim, Norway found the same bacterial genome in the dental remains of an individual buried in the 13th century. This suggests that Salmonella enterica was prevalent in Europe at least 300 years before the Spanish conquest of Mexico, reinforcing the hypothesis that it could have been transported there by asymptomatic carriers.

Is it possible that there were multiple causal agents of the Aztec epidemics?

It is quite possible that other diseases could have been involved in the ‘cocoliztli’. Findings from these previous studies are not conclusive evidence that Salmonella was the major causal agent of the decimation of the Aztec population. For example, standard symptoms of Salmonella Paratyphi C do not include gastrointestinal haemorrhage, leaving room for the distinct possibility that the ‘cocoliztli’ epidemics were a result of mixed infections.

How can researchers take a holistic approach to understanding infectious diseases of the past, present, and future?

As demonstrated across these studies, and throughout history, there is much to be learnt from the diseases which impacted our ancestors. The One Health concept allows for the visualisation of infectious diseases with a multi-discipline approach, combining cultural history with microbiology and systems biology, all alongside the relationships microorganisms have with their environment. The pathogenesis of disease and the ecology of pathogen transmission are vital not only for our understanding of the historical infections of the past, but also for us to learn how to better deal with the infections of the future.

Featured image credit: Teotihuacan, Mexico Aztec pyramids by jjnanni. Public Domain via Pixabay .

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